NO. 1 & DONE: Mark Sanchez may have been a first-round draft pick, but if he doesn’t turn it around soon, his days as a starter could be over. Just ask former No. 1 pick David Carr. Photo: Getty Images

NO. 1 & DONE: Mark Sanchez may have been a first-round draft pick, but if he doesn’t turn it around soon, his days as a starter could be over. Just ask former No. 1 pick David Carr. (
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NASHVILLE — Mark Sanchez and his perpetually dissected career will be on display again at LP Field tonight when the Jets play the Titans in a game they have to win to avoid being eliminated from playoff contention.

How much longer the Jets stay alive in the playoff hunt is less in question than how much longer Sanchez remains their starting quarterback.

Because Sanchez, whether he wants to admit it publicly or even to himself, has arrived at a significant crossroads to his career as a starter — surely much sooner than he ever imagined after being drafted as the face of the franchise four years ago.

Sanchez, a mere two years removed from playing in his second consecutive AFC Championship Game, entered this weekend ranked 34th in the NFL in quarterback passer rating. The only quarterbacks with ratings worse than Sanchez’s 71.8 rating are Matt Cassell and John Skelton, who both lost their starting jobs this season.

The primary reason Sanchez even remains the starting quarterback at this point is by default. The Jets coaches have zero confidence in either of his backups, having convinced themselves before they left Cortland that Tim Tebow cannot throw the ball and that Greg McElroy is nothing more than a decent emergency option.

So Sanchez, who is owed $8.25 million guaranteed in 2013, remains the starter. But for how long?

Sometimes salaries are guaranteed, but opportunities are not. Nor are second chances.

So if Sanchez does not reassert himself as the franchise quarterback, he could be on his way to playing himself into a backup role for the rest of his NFL career.

After all, if Sanchez does not reverse the trend of the last two seasons and is eventually let go by the Jets, what team will be banging on his door offering up a starting opportunity elsewhere?

Just down the highway to the east of the Jets training facility in Florham Park, N.J., an older contemporary of Sanchez’s, David Carr, knows all about how difficult it is to get a second chance to make a first impression.

Carr, Eli Manning’s backup with the Giants, was drafted No. 1 overall by the Texans in 2002 and he has as much perspective about the shoes Sanchez walks in as anyone, because after five seasons in Houston, management cut the cord.

Carr went from starting 76 of 80 games for the Texans in his five years there to never becoming a starter again in the league. In the five years since he left Houston, Carr has appeared in just 17 games.

From afar, he has watched the Sanchez saga unfold.

“If I had to pinpoint it I’d say their team isn’t what their team was a couple years ago,” Carr told The Post. “I don’t think that he’s a worse quarterback. It’s just that the situation around him has changed. It’s going to be interesting to see what happens. I’ll definitely be pulling for him.’’

Carr’s situation was different from Sanchez’s in that when he was drafted by the Texans, they were one of the worst teams in the league with poor talent all around him, particularly on the offensive line.

Carr was sacked 76 times his rookie year — 50 fewer than Sanchez his rookie year. Carr, too, never had the strong supporting cast Sanchez did in 2009 and 2010 before the talent level dwindled.

“If you look at Mark early on, having so much success early with the championship games, the bar was set high and they were playing good football as a team,’’ Carr said.

As much as many people want it to work out for Sanchez, the shelf life for starting quarterbacks always has an expiration date. No one knows for sure when Sanchez’s is, but as his struggles continue, that date draws closer. And when it comes, then what for Sanchez?

“Being in the position that Mark is in, you always see yourself as a starting quarterback,’’ Carr said. “That opinion is never going to change for him.’’